Strategies are a way to control play execution. By default, plays run with a linear strategy, in which all hosts will run each task before any host starts the next task, using the number of forks (default 5) to parallelize.

The serial directive can ‘batch’ this behaviour to a subset of the hosts, which then run to
completion of the play before the next ‘batch’ starts.

A second strategy ships with Ansible - free - which allows each host to run until the end of
the play as fast as it can.: